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Consolidated Statement of Changes in Stockholders' Equity (Parenthetical) - $ / shares
3 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Consolidated Statement of Changes in Stockholders' Equity [Abstract]        
Purchase of treasury stock (in shares)   17,990 66,110 49,465
Cash dividends (in dollars per share)   $ 1.67 $ 1.58 $ 1.63
Stock dividend percentage 5.00%      
Dividend paid date   Jun. 30, 2017